1. Field of the Invention
The present invention relates to a plasma reactor and, in particular, to a multiple frequency plasma reactor in which the frequencies and the powers associated therewith are individually controllable.
2. State of the Art
Semiconductor fabrication techniques are used to form integrated circuits on wafers and frequently include plasma-assisted processes for etching materials from the semiconductor wafer. Such plasma etching processes, also known as “dry etching,” are conventionally performed in a plasma reactor which utilizes radio frequency (RF) power generators to provide power to one or more electrodes within a vacuum chamber containing a gas at a predetermined pressure as defined by a specific process. The plasma reactor also includes a matching network for efficiently coupling power from the RF power generator to the electrode within the vacuum chamber.
Dry etching of a semiconductor wafer occurs within a vacuum chamber when electric fields between the electrodes within the vacuum chamber cause electrons present in the gas within the vacuum chamber to initially collide with gas molecules. With time, the electrons gain more energy and collide with the gas molecules to form an excited or ionized species. Eventually, a plasma is formed in which excitation and recombination of the atoms with electrons within the plasma are balanced. Highly reactive ions and radical species result in the plasma and are used to etch materials from the semiconductor wafer. Electric and magnetic fields within the vacuum chamber are used to control the etching processes on the semiconductor wafer.
One conventional RF-powered plasma reactor is a single-frequency diode reactor. In a single-frequency diode reactor, RF energy is conventionally applied to the wafer table on which the semiconductor wafer is located with an electrode located above the wafer serving as a grounded electrode. In such an arrangement, the plasma forms above the wafer and the ions are accelerated downward, as a result of an electric field formed between the plasma and the negatively charged wafer, into the wafer to physically etch materials from the wafer. Different frequencies presented at the electrode cause different physical phenomena in the plasma, which may or may not be desirable for a particular semiconductor process.
Another conventional RF-powered reactor includes a dual-frequency reactor which generally permits one RF frequency to be applied to a first powered electrode located away from the wafer and which predominantly controls and powers the plasma. A second RF frequency electrode provides a bias to the wafer to control the potential (e.g., sheath potential) between the second powered electrode and the plasma. Such a configuration generally assumes a capacitively coupled arrangement, which results in the formation of a self-induced DC bias to the wafer. Dual-frequency systems generally permit higher ion densities in the plasma, which results in a higher ion flux into the wafer. Such an approach significantly affects etch rates as a higher density of ions generally induces a higher etch rate.
Yet another conventional RF-powered reactor includes a dual-frequency reactor which applies two RF frequencies to a biasing electrode to control the potential between the biasing electrode and the plasma. Another electrode is located away from the wafer and is coupled to a reference potential, such as ground. The two frequencies typically perform separate functions, with one frequency dominating the ion energy while the other frequency dominates the plasma energy.
Though various arrangements for providing power to the plasma of a plasma reactor have been described, each heretofore-described configuration includes corresponding shortcomings. Therefore, there exists a need for an improved configuration which provides for a flexible solution to the foregoing problems and deficiencies.